Hard disc drives are commonly used as the primary data storage and retrieval devices in modern computer systems. In a typical disc drive, the data is magnetically stored on one or more discs that are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs. A read channel and interface circuit are provided to recover previously stored data from the discs to the host computer.
A closed loop digital servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention, is typically used to control the position of the heads relative to tracks on the discs. The tracks are defined from servo information that is written to the surfaces of the discs during manufacturing. The servo system of a disc drive thus utilizes the servo information in the performance of two primary operations: seeking and track following.
Seeking entails the movement of a selected head from an initial track to a destination track. More particularly, a velocity-control approach is typically employed wherein the velocity of the head is repetitively determined and compared to a velocity profile which defines an optimum velocity trajectory for the head as it moves to the target track. The amount of current applied to an actuator coil varies in proportion to the velocity error, the actuator coil being part of a voice coil motor used to control the position of the head.
Track following entails the continued positioning of a selected head over a corresponding, selected track. More particularly, a position-control approach is typically employed wherein the relative position of the head with respect to the center of the track is determined and compared to a desired position for the head.
A position error signal (PES) indicative of position error is generated therefrom and used to control the amount of current that is applied to the actuator coil in order to maintain the head at the desired position relative to the track.
As will be recognized, modern disc drives typically employ an embedded servo scheme wherein the servo information is angularly spaced and interspersed among user data fields (commonly referred to as "sectors") on the surfaces of the discs. However, the sampling rate of the servo information is typically insufficient to provide the gain necessary to maintain the heads within predetermined off track boundaries. Accordingly, an observer is deployed to provide estimates of head position, velocity and bias force correction at times when the heads are disposed over the user data fields. Thus, the servo system utilizes position information obtained from the discs to provide the observer with the input required to give estimates for controlling the movement of the heads during seeking and track following. Such observers (also referred to as "estimators") are well known in the art and are discussed, for example, in U.S. Pat. No. 5,585,976 issued Dec. 17, 1996 to Pham, assigned to the assignee of the present invention.
A continuing trend in the disc drive industry is to provide disc drives with ever increasing data storage and transfer rate capacities. For example, some disc drives of the current generation have track densities greater than about 3000 tracks per centimeter (about 8000 tracks per inch). The implementation disc drives with such high track densities (and corresponding high data storage capacities) places greater demands upon the disc drive servo systems in maintaining adequate control of the position of the heads in the presence of external mechanical shock and vibration applied to the drives. For example, it is important to accurately maintain the position of a head over the center of a track during a write operation to prevent the inadvertent overwriting of data on adjacent tracks. Unfortunately, traditional linear control methodologies utilized in prior art disc drive servo system designs have become increasingly deficient in providing adequate control in the presence of significant shock and vibration inputs.
An alternative control methodology known as sliding mode control (SMC) has been found to be useful in certain control applications, such as in the robotics field, wherein servo motors are used to position control objects (such as cantilevered arms) in the performance of various useful tasks. As will be recognized, SMC is a nonlinear system control methodology that is well known for its robustness in handling systems with uncertainties. However, one well known problem with SMC is the so-called "chattering phenomenon", characterized by undesirable oscillation of the control object (due to, for example, overcorrection). Additionally, as the gain of a control system is increased, sensor noise (i.e., errors between actual and measured parameters caused by inaccuracies and offsets in the measurement circuitry) typically causes undesired oscillatory response in the control object. Such detrimental effects can generally be minimized by increasing the sampling rate so that positional information is generally available to the control system on a near continuous basis. Thus, SMC has not proven to be a viable control methodology in disc drives because of the relatively low sampling rates characteristic of disc drive embedded servo designs for head positional control. For a general discussion of previous SMC applications, see for example U.S. Pat. Nos. 5,384,525 and 5,442,270 issued Jan. 24, 1995 and Aug. 15, 1995, respectively, to Kato Tetsuaki.
Accordingly, there is a continual need for improvements in the art whereby the robustness of disc drive servo systems in the presence of mechanical shock and vibration can be enhanced while accommodating ever increasing data storage capacities and transfer rate performance characteristics.